Carbon materials are generally used as cathode active materials for lithium secondary batteries that are being used in rapidly increasing number. Also, the use of lithium metals, sulfur compounds, silicon compounds, tin compounds and the like have been considered. Meanwhile, lithium-containing cobalt oxides (LiCoO2) are generally used as cathode active materials for lithium secondary batteries. Also, the use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium-containing nickel oxide (LiNiO2) as the cathode active materials has been considered.
LiCoO2 is currently used owing to superior physical properties such as cycle properties, but has disadvantages of low stability, high-cost due to use of cobalt, which suffers from natural resource limitations, and limitation of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many features associated with preparation methods thereof. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have a disadvantage of poor cycle properties.
In recent years, methods to use a lithium transition metal phosphate as a cathode active material have been researched. Lithium transition metal phosphates are largely divided into LixM2(PO4)3 having a Nasicon structure and LiMPO4 having an olivine structure, and are found to exhibit superior high-temperature stability, as compared to conventional LiCoO2. To date, Li3V2(PO4)3 is the most widely known Nasicon structure compound, and LiFePO4 and Li(Mn, Fe)PO4 are the most widely known olivine structure compounds.
Among olivine structure compounds, LiFePO4 has a high output voltage of 3.5V, a high volume density of 3.6 g/cm3, and a high theoretical capacity of 170 mAh/g, as compared to lithium (Li), and exhibits superior high-temperature stability, as compared to cobalt (Co), and utilizes cheap Fe as an ingredient, thus being highly applicable as a cathode active material for lithium secondary batteries.
However, active materials used for lithium secondary batteries require high density and rate properties. Such LiFePO4 exhibits considerably low Li+ diffusion rate and electrical conductivity. For this reason, when LiFePO4 is used as a cathode active material, internal resistance of batteries disadvantageously increases. As a result, when battery circuits are closed, polarization potential increases, thus decreasing battery capacity.
In order to solve these problems, Japanese Patent Application Publication No. 2001-110414 suggests incorporation of conductive materials into olivine-type metal phosphates in order to improve conductivity.
However, LiFePO4 is commonly prepared using Li2CO3 or LiOH as a lithium source, by solid state methods, hydrothermal methods and the like. Lithium sources and carbon sources added to improve conductivity disadvantageously cause a great amount of Li2CO3.
Such Li2CO3 is degraded during charging, or reacts with an electrolyte solution to produce CO2 gas, thus disadvantageously causing production of a great amount of gases during storage or cycles. As a result, disadvantageously, swelling of batteries is generated and high-temperature stability is deteriorated.
In another approach, a method in which a diffusion distance is decreased by reducing the particle size of LiFePO4 is used. In this case, great costs associated with the process for fabricating batteries are incurred due to high BET value.
Such LiFePO4 has a great advantage of being low cost, but having a lower density than active materials having a generally known layered structure or spinel structure due to the afore-mentioned disadvantages, thus causing a deterioration in content of active materials in the process of mixing to fabricate electrodes.
In particular, when the surface of LiFePO4 is treated with carbon (C), hydrophobic functional groups are present and further deteriorated mixing properties are thus imparted. In addition, as particle size decreases, mixing properties are deteriorated. In order to reinforce these mixing properties, the amount of solvent should be increased. As the amount of solvent increases, cracks are induced in pores formed during evaporation of the solvent in the drying process, and problems such as non-uniformity of electrodes and deterioration in conductivity are caused. Such a mixing problem is encountered in the initial process of battery fabrication, thus having a great effect on all battery processes and battery characteristics.
Accordingly, there is an increasing need for mixes that use LiFePO4 coated with carbon (C) as an active material, do not increase the amount of solvent, exhibit superior process properties and have a high solid content in the slurry.